In the practice of methods for the controlled cooling of steel rod in sequence with hot rolling for the purpose of obtaining a rod which is suitable for subsequent processing to finished product without requiring additional heat treatment, a large measure of success has been achieved with plain carbon steels in the medium-to-high carbon content range, to the extent that now many grades of plain carbon steel are being cooled directly from rolling and processed thereafter into finished wire, and other products, without requiring any heat treatment. The process by which this is accomplished is described in the McLain et al. U.S. Pat. No. 3,231,432. It involves hot-rolling the rod and directly thereafter coiling it onto an open conveyor in spread-out ring form while the microstructure of the steel is still in a condition of highly uniform, relatively small austenite grain size, and then cooling it rapidly through allotropic transformation by the application of a moving air stream to the rod while the rod is still spread out.
Certain alloy steels and low carbon steels, however, cannot tolerate the high cooling rates employed in the typical practice of the above-cited McLain process. For instance, a cooling rate of as low as 0.2.degree.C/sec. is sometimes required. However, although the desirability of providing for such low cooling rates has been known for many years, and equipments have been available for 6 to 8 years in which the average cooling rate is sufficiently slow, a number of hitherto unsolved problems have been presented. Uniformity of cooling in all parts of the rod is the major problem, and the equipments such as ovens and insulated hot boxes which were capable of achieving those slow average cooling rates simply were incapable of producing the desired rod quality. The reason for the non-uniformity was not altogether apparent, and its explanation requires an understanding of various background factors. For instance, the conditions of rod delivery pose the initial problem. Modern high speed rod rolling mills deliver the rod at over 10,000 fpm (50 meters/sec.), at which speeds the rod cannot be handled in straight lengths. It must be coiled. If it is coiled into a bundle according to the universal practice before the above-cited McLain process, the resulting bundle represented a relatively confused mass. Uniform cooling was impossible with such bundles. One might suppose that such a bundle would cool very slowly uniformly, but in fact it does not when exothermic allotropic transformation of steel is involved. Coiling the rod onto a moving conveyor in spread-out ring form as in the McLain process reduced the confusion of the bundle, but it still left the rod mass somewhat more compacted at the sides of the conveyor than at the center. Obviously, non-uniform cooling rates resulted from this, but the interesting feature of the McLain process was that when medium to high plain carbon steels were rolled, the uniform and small austenite grains transformed so rapidly that the non-uniform cooling rates which inherently resulted from the ring configuration, did not have time enough to result in vastly different average temperatures for transformation. The result was to provide a rod which had adequately uniform physical properties in spite of the non-uniformity of cooling on the conveyor. This fortuitous aspect of the McLain process, however, diminishes rapidly when the time of transformation is increased. Normal good cooling rates for medium to high carbon steels with the McLain process are about 7.degree.C to 11.degree.C/sec. or higher. Non-uniformity and poor quality are noted increasingly as the rate drops below 7.degree.C/sec. Of course, many alloy steels and some low carbon steels require cooling rates very much lower than that in order to achieve the desired properties.
Several attempts have been made to coil the rod rings concentrically on both vertical and horizontal axes as, for example, in U.S. Pat. 3,494,603. Theoretically some improvement of uniformity of cooling ought to be accomplished thereby. However, nothing significant has been noted or published in this connection. With such equipments it is inevitable that the rod must rest on supports or on adjacent rings, and therefore, any improvement in uniformity for very slow cooling which might be obtained by using such equipments has not materialized. Moreover there are substantial disadvantages and complexities involved in the use of those equipments.
Another problem related to slow cooling concerns the various forms of heat loss and the ease of their respective controllabilities. The two principal forms of heat loss in the cooling of hot rolled rod are radiation and convection. At rolling temperature, about 1850.degree.F (1000.degree.C), the rod loses heat primarily by radiation. But when it has cool to the transformation range of approximately 1400.degree.F (700.degree.C), radiation plays a less prominent role (radiation heat loss decreases inversely as the 4th power of the difference in temperature Kelvin between the radiating body and the absorbing body). Thus, cooling by radiation and with only a minor amount of convection, 5.5 mm rod at 1400.degree.F (700.degree.C) will cool at an average rate across the conveyor of about 2.degree.C/sec. Of course, forced air convection can be applied to increase the rate to values above 11.degree.C/sec, but the point relative to slow cooling below 2.degree.C/sec. is that the entire cooling can be done radiantly. This permits the use of a process which suppresses convection as much as possible and regulates the cooling rate by controlling the radiant cooling only.
Experience shows that when rod is laid out on a simple flat conveyor with minimal convection, the edges of the rings cool at about 1.6.degree.C/sec. to 2.1.degree.C/sec., and the centers cool at about 2.3.degree.C/sec. to 2.8.degree.C/sec. depending upon the spacing of the rings. When slower rates applicable for given grades of steel are desired, placing the rings in an insulated box or oven can accomplish it but with such equipments, the same general relationship remains between sides and center and the cooling is non-uniform. This may be satisfactory for some products, but much greater uniformity is required in many cases, particularly for high alloy steels. In fact greater uniformity of cooling than is achieved with the usual practice of the above-cited McLain process, is even required in some plain high carbon grades.
Another major problem is the provision of equipment which is not only suitable for slow cooling but which can also be converted rapidly and conveniently to high speed cooling. Even though uniform, very slow speed cooling is desirable in some limited cases, high speed cooling still must be used for the major tonnage of steel rod that is rolled. Therefore, it is highly unlikely that a modern high speed rod mill will ever be built for very slow speed cooling alone. Therefore a successful installation for very slow speed cooling must not only provide uniformity of cooling but it must also be capable of conveniently changing from slow speed cooling to high speed cooling.
Another requirement is certainty of performance. When rod is being drawn into wire, a very high premium is placed or drawability without breaks. For example, if a small percentage of the rod, in excess of predetermined percentages is sufficiently bad to cause breaks in wire drawing at random locations throughout the length, the whole bundle becomes a reject. Of course, breaks in wire drawing are not the only problem. The physical properties of the finished product must also meet the specifications. Moreover, the process must be capable of producing good rod, bundle after bundle. If one bundle out of very five is a reject, and no one can tell in advance which one is the bad bundle, the whole production is suspect. Thus in the cooling of hot rolled rod, a very high drgree of perfection is required if the object is to avoid further heat treatment. This is why the above-cited McLain process is so successful. When it is used for plain, medium to high carbon steels, its product meets the required specifications virtually 100% of the time, and it accomplishes this result without requiring the operators to exercise any particular skill in the control. In the same way a process for very slow cooling must be equally sure, simply in its control, and repetitive in its results.
Accordingly the primary object of this invention is to provide an apparatus and a process for very slow speed cooling which accurately controls the cooling rate throughout the length and cross-section of the rod. It is also an object to make such an apparatus and process readily convertible to high speed cooling and to achieve an infinite control of cooling rates between substantially 0.degree.C/sec. and 20.degree.C/sec. In addition, an objective is to make its operation highly certain and repetitive in its performance once it is initially adjusted.